/ llr. I R .\ A Nr ( )/{ .M I: I! _( I*. t 



Although diagrammatically the auto-transformer looks like 

 a drop wire, its operation is entirely different. The auto-trans- 

 former is superior, both as regards efficiency and voltage 

 regulation. 



Figure 198 (a) shows a regular transformer, which transforms 

 1,500 watts from 100 volts and 15 amp. to 75 volts and 20 amp. 

 That is. the voltage is stepped down in the ratio of 4 to 3. The 

 primary current l ac is 15 amp., and the secondary current I c >v is 

 20 amp. as shown. When the windings be and b'c' are combined 

 to make an auto-transformer, Fig. 198 (6), the net current in be 

 is hut 5 amp. The winding b'c' in (a) may be eliminated entirely 

 and winding be in (6) need be only one-third the cross-section of 



Ao1 * - ,.,. 



+-16 Anif. C C' ~* 20 Amp. + 15 Amp. C -* 20 Amp. 



(a) Regular Transformer ( ) Auto-Transformer 



Fi<;. Hs. Currents and voltages in an auto-transformer supplying load at 75 



per cent, voltage. 



i he winding be in (a). Hence there is a very considerable saving 

 in copper in the auto-transformer over the regular transformer. 



In Fig. 198 (6), 



Primary power in ab = 25 X 15 = 375 watts, 

 ondary power in be = 75 X 5. = 375 watts. 



Transformed power = 375 watts. 



Power conducted must then he 1,500 - 375 = 1,125 watts. 

 Only nnf-fourth the total power involved is now tninsfo 

 whereas in Fig. 107, one-half the total power was transformed. 



The auto-transformer is sometimes called a compensator. 

 It is a type of transformer which transforms a portion of the 

 energy and allows the remainder to How conduct ivelv through 

 iis windings. Its action is analogous to the balancer set. 

 Vol. I .'!, p : ,r. iM!).i The current / in dropping ti- 



the current / > to the rolUg / . \Vind- 

 poodfl to the motor and winding !>< to the generator 

 J. Vol. I. page 391. In fact, a compensator can be used 



